The present invention relates to the production of optical spatial filters, and more particularly, to the production of such filters for use in inspecting IC photomasks and wafers with a holographic optical processor.
As is well known in the art, an optical Fourier transformation can be performed on a subject by illuminating the subject with a coherent light source and using a lens to image the subject at infinity. The Fourier transform of the original subject information will occur in a plane located at the focus caused by the lens. With this process, subject information is redistributed in the Fourier transform plane to correspond to spatial frequency content. If the subject consists of repetitive spatial frequency components such as is present on photomasks used in the production of microelectronic circuits, the optical Fourier transform will consist of an array of regularly spaced points of light whose distance from the optical axis is proportional to the spatial frequency.
By placing a blocking filter that is opaque at each of these points within the Fourier transform plane, and then performing an inverse transform, the resulting image contains only nonrepetitive information about the original subject. For example, nonrepetitive defects in the original photomask or wafer would pass this spatial filter and be reimaged, but repetitive circuit information would not. This technique offers a potentially powerful tool for locating critical defects in complex photomasks and wafers. Producing an accurate blocking filter, however, has proven to be a difficult task for several reasons.
First, it is known that the location and size of the necessary opaque regions in the filter can be accurately calculated according to well known optical principles, allowing for the production of a filter by computer generation or other synthetic means. Such a technique is relied upon, for example, in U.S. Pat. No. 4,000,949, issued Jan. 4, 1977 to Watkins, and in U.S. Pat. No. 3,738,752, issued June 12, 1973 to Heinz et al. As is recognized in Heinz, however, any aberrations and distortion present in a practical optical system must be accounted for in generating the filter. Therefore, distortions and aberrations in the optical system must be carefully controlled, thus making the system optics extremely expensive and/or reducing the overall performance to less than ideal.
Other approaches include producing filters with increased opaque areas, such as is disclosed in U.S. Pat. No. 3,658,420, issued Apr. 25, 1972 to Axelrod, U.S. Pat. No. 3,972,616, issued Aug. 3, 1976 to Minami et al, and U.S. Pat. No. 3,790,280, issued Feb. 5, 1974 to Heinz et al. Filters produced by these techniques have the advantages of improved universality and noncritical alignment. They also, however, have the disadvantage that the overall transmission to nonrepetitive or defect information is lower, thereby decreasing the detectability of the defects within the subject. In addition, these simple geometry filters consist of opaque crosses, x's, pie-shaped wedges, and variations or combinations of these with or without opaque dots. As a result of these simple and general geometries, the filters do not exactly match the subject information. This leaves a residual background image of the subject, resulting in a low signal-to-noise ratio, and makes automating the analysis of the output of the optical processor much more difficult.
A third alternative technique would be to record the actual location of the points in the optical Fourier transform plane corresponding to repetitive or non-defect subject information on a photographic film or plate. This would seem to allow for an accurate replication of information present in the transform plane, except that the typical dynamic range of intensities within the transform plane can be on the order of 10.sup.10 or higher. Photographic emulsions and other photosensitive materials, however, are typically restricted to dynamic ranges only on the order of 10.sup.3 to 10.sup.4. Because of this limited dynamic range, the entire information in the Fourier transform plane cannot be accurately recorded.
What is needed, therefore, is a technique for producing a high performance optical spatial filter that avoids the problems set forth above. In particular, the filters produced by such a technique should enable an optical processor to be capable of detecting defects within the subject photomask or wafer of a relatively subtle nature and isolating those defects in a manner having a relatively high signal-to-noise ratio.